A tap changer having in total four vacuum switching tubes per phase is known from DE 20 21 575. Provided in each of the two load branches are a respective vacuum switching tube as main switch and a respective further vacuum switching tube, in series connection with a switch-over resistance, as resistance switch.
In the case of uninterrupted load changeover from the previous winding tap n to a new, preselected winding tap n+1 initially the main switch of the side switching off is opened and thereupon the resistance switch of the side taking over closes so that a compensating current limited by the switch-over resistors flows between the two taps n and n+1. After the previously closed resistance switch of the side switching off has opened, the main switch of the side taking over then closes so that the entire load current is conducted from the new winding tap n+1 to the load concluding the changeover.
A further tap changer in which further mechanical switch elements are provided between the electrical connection of the two vacuum switching tubes of each load branch and the load is described in DE 10 2009 048 813 that is not prior published.
The known tap changer requires four separate vacuum switching tubes per phase. In the first instance, the large amount of space required for these vacuum switching tubes themselves, as well as the associated actuated mechanism is disadvantageous. Moreover, due to the high component outlay such known constructions are relatively expensive.